The present invention relates to a composition for charge transport layer and an electrophotographic member prepared using the composition.
Since electrophotographic members using organic photoconductive compounds are advantageous in flexibility, light weight, surface smoothness and price, recently they are widely studied. Among them, function separation type electrophotographic members having a charge generation layer wherein a charge carrier is formed by absorbing light and a charge transport layer wherein the charge carrier formed is transported by an electric field can remarkably improve photoresponsiveness and sensitivity which have been inferior in conventional electrophotographic members in which organic photoconductive compounds are used. Thus, the function separation type electrophotographic members have recently showed rapid progress. These function separation type electrophotographic members are mounted on electrophotographic devices (printers, copying machines, etc.) to which the Carlson method is applied.
However, with recent demand for enhancement in quality of printed images obtained by electrophotographic devices such as copying machines and laser beam printers and increase in printing speed owing to miniaturization of electrophotographic devices, electrophotographic members are further strongly required to give printed images of high quality and have rapid photoresponsiveness.
Hitherto, as binder resins for the charge transport layer of electrophotographic members, bisphenol A type polycarbonate resins represented by the following formula have been most commonly used from the points of transparency and mechanical strength. ##STR2##
Compositions for charge transport layer are prepared by homogeneously dissolving or dispersing in a solvent a charge transporting substance, a bisphenol A type polycarbonate resin as a binder resin and if necessary, additives such as plasticizers, flowability imparting agents and pin hole inhibitors.
However, as can be seen from the above formula, since bisphenol A type polycarbonate resins are inferior in solubility, halogen solvents such as methylene chloride, 1,2-dichloromethane and 1,1,2-trichloroethane are used each alone or in admixture or mixed solvents of halogen solvents and non-halogen solvents are used.
Furthermore, for obtaining a high photoresponsiveness, it is necessary to increase the drift mobility. For this purpose, usually the amount of the charge transporting substance in the composition for charge transport layer is increased.
In the composition for charge transport layer in which content of the charge transporting substance is increased, when this is in the form of a solution, the charge transporting substance and bisphenol A type polycarbonate resin are uniformly dissolved, but when this is dried and the solvent is removed to form a solid charge transport layer, the charge transporting substance and the bisphenol A type polycarbonate resin separate from each other and the coat tends to become ununiform in both the form and the composition. When electrophotographic members are prepared using such composition, defects in images such as fogging, black points and white stains occur from the initial stage of use. Thus, electrophotographic members which satisfy the high photoresponsiveness and the high image quality have not yet been obtained. On the other hand, movement for the environmental protection of the earth has become active and abolition of the use of freon which destroys the ozone layer has been demanded and regulations for use of halogen solvents which contaminates underground water have been strengthened.
As polycarbonate resins soluble in non-halogen solvents, there have been known bisphenol Z type polycarbonate resins having the recurring unit represented by the following formula: ##STR3## However, when these bisphenol Z type polycarbonate resins are used, there is a problem in that even when amount of the charge transporting substance in the charge transport layer is increased, the drift mobility cannot be increased, and when the charge transporting substance is contained in a large amount for obtaining high photoresponsiveness, there occurs a problem in that the film strength of the charge transport layer decreases.